Geology in space

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The marks on Mars

Looking at Earth, Venus, and Mercury so far has shown different systems of tectonics. The last terrestrial planet Mars has some familiar features but all is not as simple as it seems.

Mars Magnetic crust as seen by Mars Global surveyor (Nasa/JPL)

Magnetic images of Mars show band like repetitions of positive and negative anomalies at its southern hemisphere. These bands look similar to magnetic bands seen in Earth’s oceanic crust.

Bands of magnetic anomalies in the crust of the Atlantic ocean

The bands on Earth’s ocean floors represent the gradual formation of crust over time; as the plates move apart and magma rises up to form new crust it records the magnetic field as it forms, over time the periodic flipping of the Earth’s magnetic field leads to an alternating pattern.

Mars doesn’t currently have an active magnetic field, and crater dating shows that the south is older than the northern 1/3 of the planet which is topographically 3-6 km lower than the southern portion of the planet. This northern lowland does not show magnetic banding. Theories suggested for this lowland include proto-tectonics like that on Earth but which did not occur until after the planet’s magnetic field stopped and/or large impacts causing the substantial topographic difference between the two regions.

Another feature which provides clues about the nature of Martian tectonics is Tharsis Plateau, this is a vast volcanic system close to the equator and includes the 22 km high Olympus Mons, the highest volcano in the solar system. This volcano was able to grow much bigger than the hotspot volcanoes seen on Earth due to a lack of movement, if the plates had been moving then the volcano would not have developed to the same extent. This shows that Mars the plates were not moving unlike on Earth.

Olympus Mons (JPL)

The huge weight of the plateau put a lot of stress on the crust around it and led to the formation of Valles Marineris. The extra load on the crust mean that large valleys formed close to the edge of Tharsis Plateau as the weight caused the crust to buckle and break and shear.

Valles Mariners, a crack caused by the weight of nearby volcanic plateau (NASA)

The magnetic bands suggest that early in Martian history it may have had some form of spreading ridge generating crust in a magnetic field. Both of these processes stopped as Mars cooled. These bands and the north-south topographic divide hint at an early active tectonic history, however, there are few signs of large-scale subduction which would allow recycling of plates so it seems unlikely that Mars ever developed a fully functioning multiple plate tectonic system, though an understanding of its history is far from complete.